Problem: Which of the following numbers is a factor of 66? ${3,5,10,12,14}$
By definition, a factor of a number will divide evenly into that number. We can start by dividing $66$ by each of our answer choices. $66 \div 3 = 22$ $66 \div 5 = 13\text{ R }1$ $66 \div 10 = 6\text{ R }6$ $66 \div 12 = 5\text{ R }6$ $66 \div 14 = 4\text{ R }10$ The only answer choice that divides into $66$ with no remainder is $3$ $ 22$ $3$ $66$ We can check our answer by looking at the prime factorization of both numbers. Notice that the prime factors of $3$ are contained within the prime factors of $66$ $66 = 2\times3\times11 3 = 3$ Therefore the only factor of $66$ out of our choices is $3$. We can say that $66$ is divisible by $3$.